/Engine/Plugins/Enterprise/DatasmithImporter/Source/DatasmithImporter/Public/ObjectElements/DatasmithUSceneElement.h
Physical Sky could be generated in a large amount of modes, like material, lights etc that's why it has been added as static, just enable it and it is done. Notice that if a HDRI environment is used this gets disabled.
[UFUNCTION](Programming/UnrealArchitecture/Reference/Functions)(BlueprintCallable, Category="Datasmith | Scene") bool GetUsePhysicalSky() const